The subject matter of the present invention relates within a to x-ray tubes employing heated filament cathodes contained within focusing cup electrode, and in particular to such x-ray tubes in which a layer of high work function material is coated on the focusing cup to prevent the field emission of electrons therefrom.
Thermionic x-ray tubes having rotating anodes are operated at extremely high voltages, typically on the order of 100 kilovolts, so that there is a tendency for the focusing cup to emit electrons by field emission to the anode or to the cathode filament when the cup is negatively biased relative to such filament. This is a particular problem in condenser discharge x-ray systems and other systems employing coaxial cables or transformers of high secondary capacitance because the high voltage is stored in the capacitance across the x-ray tube so that any field emission from the focusing cup causes a high current discharge. The field emission of electrons from the focusing cup to the filament cathode can destroy such cathode, particularly if it is a thoriated tungsten filament cathode which is easily damaged by evaporation of thorium from the filament or contamination of the filament by the deposit of evaporated material from the focusing cup. In addition, during manufacture the unactivated filament cathode may be damaged by field emission from the focusing cup during "seasoning" as described in copending U.S. patent application Ser. No. 228,951, filed Feb. 24, 1972, by Z. J. Atlee et al. .Iadd., now U.S. Pat. No. 3,846,006.Iaddend..
These problems are avoided in the x-ray tube of the present invention by employing a non-emitting coating of high work function materials, such as platinum or gold, on the surface of the focusing cup electrode including the inner surface portions within the cup which are immediately adjacent to the cathode filament. In the preferred embodiment, a platinum coating is employed because of its higher work function and higher permissible operation temperature. The platinum is applied to the focusing cup by sputtering or ion plating which avoids melting the underlying focusing cup metal which would happen if a fusion coating method were employed due to the high melting point of platinum. Previously, it has been suggested .[.tht.]. .Iadd.that .Iaddend.the anodes of high voltage rectifier tubes can be coated with a thin layer of gold over a thicker intermediate nickel layer provided on such anodes by electroplating and subsequent heating below 780.degree. Centigrade, as discussed in U.S. Pat. No. 3,611,523 of E. S. Den Dulk, patented Oct. 12, 1971. However, heating above this temperature causes a low melting temperature alloy of gold and nickel to form which no longer has a high work function. However, this is impractical for coating the focusing cup of an x-ray tube because frequently such focusing cup is processed and operated at higher temperatures.
It is, therefore, one object of the present invention to provide an improved x-ray tube of longer useful lifetime in which the focusing cup electrode is coated with a non-emissive layer of high work function material.
Another object of the invention is to provide such an x-ray tube in which the coating of non-emissive material is applied to the focusing cup by a method which maintains the high work function of the material, results in good adherence and provides a smooth surface on such focusing cup.
Still another object of the present invention is to provide such a method in which the low emissive material is applied to the focusing cup base material by sputtering.
A further object of the invention is to provide such a method using ion plating.
A still further object of the invention is to provide such a method in which a low emissive material is applied to the focusing cup by electroplating followed by fusion.
An additional object of the present invention is to provide the focusing cup electrode of an x-ray tube with such non-emissive coating of a high melting point material, such as platinum, without melting the underlying base material.